What should be done if the jet pump on a cottage water system (which feeds into a bladder-type pressure tank) comes on when a tap is opened and stops when the tap is turned off?
It sounds like a problem inside the pressure tank, the part of the system that holds the water and maintains its pressure. It contains a rubber bladder (or diaphragm), filled with water under pressure which squeezes out of the tank when you open a tap. As the water drains, a gauge—similar to one on a bicycle pump—continually monitors whether pressure in the tank has dropped below a set level. Once it does, the pump starts and water refills the bladder.
When a pump immediately switches on and off while turning on a tap or flushing the toilet, the pressure tank is waterlogged. A tear or hole in the bladder caused by use and age has almost certainly allowed water to seep into the air chamber inside the tank, displacing some or all of the air. When there’s no air to push on the water, the pump works overtime to compensate by bringing water in to restore the pressure, which is why the pump turns on every time the tap is opened. These frequent starts and stops—known as “short cycling”—can burn out the motor and damage the pressure switch. Draining the tank won’t help. While bladders can be replaced in a few, higher-end tanks, the best solution for the basic systems found in most cottages is to simply replace the pressure tank.